NOx in the atmosphere leads to a photochemical smog or acid rain. Accordingly, discharge of NOx from cars or the like equipped with an internal combustion engine that is one of the NOx generating sources becomes a social problem. Therefore, it has been under consideration to make laws and regulations strict for discharge of NOx in the future.
Although a diesel engine or a gasoline lean burn engine is advantageous from a viewpoint of fuel efficiency, such an engine is known to have a large amount of NOx generated. This is because such an engine is in a lean burn manner in which a leaner air-fuel mixture compared to the theoretical air/fuel ratio is burned, so that when the combustion temperature reaches high temperature, large amounts of nitrogen and oxygen included in the air react with each other, etc. In addition, it is difficult to sufficiently reduce NOx because a lot of oxygen is included also in an exhaust gas in a lean atmosphere.
Conventionally, various NOx purification technologies have been proposed. For example, a method for absorbing and treating NOx is proposed, including causing a catalyst to absorb NOx in a lean atmosphere, and subsequently introducing CO or HC by engine control to temporarily bring about a reducing atmosphere (a rich atmosphere), thereby purifying the absorbed NOx (Patent Literature 1). Furthermore, as a technology to make up for the disadvantage of the method for absorbing and treating NOx, a method is proposed, including placing a catalyst for oxidizing HC and CO at the upstream side of an exhaust gas flow, and providing a NOx purification catalyst at the downstream side thereof (Patent Literature 2). In addition, a technology to effectively utilize hydrogen (H2) included in an exhaust gas is disclosed, including selectively purifying HC and CO at the upstream side of an exhaust gas flow, and purifying NOx with the use of the remained H2 at the downstream side thereof. Moreover, a technology is proposed, including actively introducing a reductant such as ammonia into an exhaust gas, thereby purifying NOx (Patent Literature 3). In addition, an exhaust gas purification catalyst is disclosed, having high NOx purification performance in the operation range, in particular under low temperature condition (Patent Literature 4). In addition, an exhaust gas purification catalyst having high NOx purification performance is disclosed, in which ceria (cerium oxide (IV); CeO2) having a NOx adsorption capacity is combined with zeolite having an HC adsorption capacity (Patent Literature 5). In addition, an exhaust gas purification catalyst having three coated layers is disclosed, the outermost layer of which is provided with a layer composed of ceria alone to suppress poisoning by SOx (Patent Literature 6).